1. Field of the Invention
The present invention relates to a multichannel display data generating apparatus which generates data for displaying AV data on a multiscreen comprising a plurality of screens displaying a plurality of channels of AV (audio-visual) data, a medium, and an informational set.
2. Description of the Related Art
Digital broadcasting has been conducted in various broadcasting forms such as BS (broadcasting satellite) broadcasting, CS (communications satellite) broadcasting, and CATV (cable television) broadcasting. These forms of broadcasting are generally conducted in multiple channels. Thus, there is broader option for selecting a program to be viewed.
Such circumstances has accelerated the spread of multichannel display in which a television screen for viewing programs comprises a plurality of screens which display a plurality of channels of AV data simultaneously.
Typical multichannel display comprises a main screen for displaying the video image of an AV data the voice of which is output from a main speaker and a subsidiary screen for displaying the video image of another AV data the voice of which is not output from the main speaker. The multichannel display permits the viewing of video images being broadcast simultaneously in a plurality of channels. For example, while watching a movie displayed on the main screen, it is possible to view a broadcast of a baseball game displayed on the subsidiary screen thereby to seize the progress of the game.
As such, the multichannel display permits the simultaneous viewing of programs being broadcast in a plurality of channels. Thus, it is concluded that the multichannel display is a programs-displaying method suitable for multichannel broadcasting.
A prior-art multichannel display data generating apparatus for generating data for performing such multichannel display is described below with an example of an STB (set-top box).
FIG. 17 is a block diagram of the configuration of an STB for generating data for performing multichannel display. FIG. 18 illustrates an example of a television screen which displays the data for performing multichannel display generated by the STB of FIG. 17.
In FIG. 18, a television screen 103 consists of four screens. A main screen 104 is a screen for displaying a video the voice of which is output from a main speaker. Subsidiary screens 105, 106, 107 are screens for displaying videos the voices of which are not output from the main speaker.
Returning to FIG. 17, an STB 100 comprises a BS tuner 4, an IEEE1394 interface 5, a transport decoder 101, and an AV decoder 102.
The transport decoder 101 includes four PCR extracting sections 11a-d and four PLL (phase-locked loop) sections 12a-d. The AV decoder 102 includes four STC (system time clock) counters 13a-d. 
The BS tuner 4 is means for receiving a BS broadcast. The IEEE1394 interface 5 is means for exchanging data and commands with external devices through an IEEE1394 bus 3 on the outside of the STB 100. The transport decoder 101 is means for separating MPEG2 transport stream transferred via the IEEE1394 bus. The AV decoder 102 is means for expanding MPEG2-compressed AV data and generating analog signals.
The PCR extracting sections 11a-d are means for extracting PCR (program clock reference) from the AV data of each channel to be displayed on each screen of the television screen 103. The PLL sections 12a-d are means for establishing PLL synchronization using the PCR extracted by the PCR extracting sections 11a-d. In other words, these are means for synchronizing the oscillation frequencies with those of the oscillators of 27-MHz frequency used by the encoders in the broadcasting stations as a time standard during the encoding of the data. The STC counters 13a-d are means for reproducing the times of day used by the encoders in the broadcasting stations during the encoding of the data, using the 27-MHz clock signals obtained by the PLL sections 12a-d and the PCR values extracted by the PCR extracting sections 11a-d. 
The STB 100 is connected to an antenna 6. The IEEE1394 interface 5 is connected to the IEEE1394 bus 3, to which a CS tuner 7 and a DVHS 8 are also connected.
The operation of such a prior art STB 100 is described below.
Broadcast waves transmitted from broadcasting stations are converted to electric signals by the antenna 6. The BS tuner 4 then receives and demodulates the broadcast waves having been converted to the electric signals. Here, program data is sent using the transport packets of MPEG2 transport stream.
The demodulated data is transferred to the transport decoder 101.
The transport decoder 101 separates the MPEG2 transport stream. At that time, the transport decoder 101 performs the following process.
That is, the transport decoder 101 extracts PCR from the AV data of each channel to be displayed on each screen of the television screen 103. The television screen 103 comprises four screens in total of the main screen 104 and the three subsidiary screens 105-107. Thus, PCR is extracted from the AV data of the channels to be displayed on these four screens.
More particularly, the PCR extracting section 11a extracts the PCR of the AV data to be displayed on the main screen 104. The PCR extracting section 11b extracts the PCR of the AV data to be displayed on the subsidiary screen 105. The PCR extracting section 11c extracts the PCR of the AV data to be displayed on the subsidiary screen 106. The PCR extracting section 11d extracts the PCR of the AV data to be displayed on the subsidiary screen 107.
The PLL sections 12a-d then establish PLL synchronization, using the extracted PCR of the respective channels. That is, the PLL sections synchronize the oscillation frequencies with those of the oscillators of 27-MHz frequency used by the encoders in the broadcasting stations as a time standard during the encoding of the data. The PLL section 12a establishes PLL synchronization with the channel to be displayed on the main screen 104. Similarly, the PLL sections 12b, 12c, 12d establish PLL synchronization with the channels to be displayed on the subsidiary screens 105, 106, 107, respectively.
The STC counters 13a-d reproduce the times of day used by the encoders in the broadcasting stations during the encoding of the data, using the 27-MHz clock signals obtained by the PLL sections 12a-d and the PCR values extracted by the PCR extracting sections 11a-d. That is, the STC counter 13a reproduces the time of the channel to be displayed on the main screen 104. Similarly, the STC counters 13b, 13c, 13d reproduce the times of the channels to be displayed on the subsidiary screens 105, 106, 107, respectively. Using these STC counters 13a-d and establishing AV synchronization with respective channels, the AV decoder 102 expands the compressed data in MPEG format and converts them to analog signals. That is, in the timing that the PTS (presentation time stamp) described in the data coincides with the time indicated by the STC counter 13 of that data channel, the part of the data is expanded and converted to analog signals.
The analog signals obtained by the above-mentioned process are output as a video signal and an audio signal. Video images are displayed on the four screens: the main screen 104 and the three subsidiary screens 105-107. The voice of the channel displayed on the main screen 104 is output from the main speaker.
As such, the prior art STB 100 have established AV synchronization with each channel independently using the PCR of the channel to be displayed on each screen of the multiscreen. That is, each channel was individually provided with a PCR extracting section, a PLL section, and an STC counter, and the process was performed on an each-screen basis. Accordingly, for a multiscreen consisting of four screens, the process was performed by four sets of PCR extracting sections, PLL sections, and STC counters, each set being provided for each channel.
However, since a prior-art multichannel display data generating apparatus comprises the same number of the sets of PCR extracting sections, PLL sections, and STC counters as the number of the screens the AV synchronization of each of which is to be established independently, there is a problem that the scale of the circuitry of PCR extracting sections and PLL sections becomes large and that the AV-synchronization control becomes complex.